It is known that certain semiconductor wafer monitoring processes utilize mass spectrometers or other apparatus in order to determine the presence and relative amount of process gases. A number of these processes, such as those, for example, utilizing Chemical Vapor Deposition (CVD) techniques, contain volatile silicon and/or other species which can cause a mass spectrometer monitoring the process to lose sensitivity in a relatively short period of time; that is, as compared to the average lifespan of an ion source typically used in conjunction with the spectrometer. More succinctly, the resulting problem that ensues is that the ion source can lose required sensitivity in a matter of days, as opposed to the normal or typical lifetime (e.g., months) of the ion source, thereby necessitating premature replacement of same.
This loss in sensitivity noted above is attributable to the accumulation of insulating deposit on the interior of the anode of the ion source. Typical ion sources are depicted in FIGS. 1 and 2, while a mass analyzer system 31 incorporating same is illustrated in FIG. 3. For purposes of better describing the problems, reference is made to each of these Figs.
First and with regard to FIGS. 1 and 2, a pair of ion sources 10, 30 is shown. Components commonly used in each of these sources and referred to herein are labeled with the same reference numerals for the sake of clarity.
As to the differences between the depicted ion sources 10, 30, some ion source manufacturers have used replaceable anodes in which the whole element is replaced or removed for cleaning, such as those, for example, in an ion source that was manufactured by Leybold Inficon of East Syracuse, N.Y. for their Q-Mass sensor system. Typically, these organic mass spectrometer units have gas entry extending from a gas chromatograph or other form of output that enters the side of the anode (i.e., laterally),as shown in FIG. 1, representative of a portion of the known ion source 10.
However and for vacuum processing applications, process analyzers based on residual gas analyzers (RGAs) such as the Compact Process Monitor manufactured by Inficon, Inc., typically have a closed ion source 30, such as shown in FIG. 2.
Each of the ion sources 10, 30 commonly include an electron stream producing means, in this case a heated filament 14, typically made from tungsten or a similar material that forms an electron stream which projects into the structure of the anode 18, 32, respectively. As noted above, the anode 18 according to the ion source 10 of FIG. 1 is replaceable, the anode being shown in both the assembled and unassembled positions in the figure, while the closed ion source 30 of FIG. 2 includes a fixed anode 32 with supporting structure such as a sealed disk 34 at the upper end thereof.
Electrons that are formed from the heated filament 14 of each ion volume 10, 30 are expelled into an ionization volume or region within the interior of the anode 18, 32. The potential of the anode 18, 32 is positive with respect to the filament and an electron repeller (not shown). Reagent gases from a deposition chamber or other source to be monitored are provided into the ionization volume. As noted above and in the instance of the ion source 10, the gases are provided laterally through a port 22 while in the ion source 30, the gases are provided axially; that is, the gases are introduced in a direction 27 that is substantially perpendicular to the direction of the electron stream through the anode 32.
An example mass analysis system 31 is shown in FIG. 3 in which a sensor 33, that houses the ion detector and Quadrupole mass detector, is arranged relative to a vacuum test chamber 35 and a vacuum pump 37 that draws the reagent gases into the ionization volume. Gas, from process 20 is supplied to the closed ion source 30 by means of a flow control orifice 21. Additional details concerning the above system are provided in U.S. Pat. No. 5,889,281, the entire contents of which are herein incorporated by reference.
In each ion source 10, 30, the ions resultingly formed in the confines of the ionization volume are pulled by appropriate potential through an ion lens assembly that comprises at least one focus plate or extractor 24 and a parallel and concentric exit lens 29. The plate 24, having less positive potentials to that of the anode 18, 32, serves to accelerate the formed positive ions as a focused ion beam 26 through concentric openings 28 in the ion lens assembly along an axis 25 to a mass filter or other apparatus (not shown in FIGS. 1 and 2), such as a quadrupole. Insulators 38 are provided in the lens assembly of each ion source 10, 30 to prevent gas leakage. In quadrupole mass spectrometers (hereinafter referred to QMS) especially, the sensitivity (that is, the ion current that is detected in ratio to the ion source partial pressure) is extremely dependent upon ion energy.
In either instance, the electron beam heating the anode surface can induce the formation of an insulating deposit layer 39 from the CVD reagent gases that are being monitored. Subsequently, the same electron beam accumulates electrons on the insulated deposit layer surface 39, forming a negative surface charge and generating an electrical potential that is negative with respect to the anode.
Typically, a closed ion source 30, such as shown in FIG. 2, that is used for process monitoring is operated to produce ions with approximately 6–8 electron volts of ion energy. The ion energy of the resulting ions entering the mass analyzer (not shown in FIG. 2)is reduced by the negative potential that is produced by the insulating layer effect described above, drastically reducing sensitivity for closed ion source QMS units.
There are two traditional solutions for solving the above problem that are currently practiced in accordance with the known art. The first solution is a total replacement of the ion source. This solution is extremely expensive in that the ion source includes a number of components in addition to the anode. This first solution is also time consuming. The second solution is replacement of the standard anode. The latter solution requires a disassembly of the ion source in addition to a replacement of the anode. In all likelihood, the latter solution also requires a replacement of the filament, thereby incurring additional repair costs.
In the ion source 10, the side or lateral entry of reagent gas through port 22 lends itself to removal of the anode 18 along the axis 25 of the ion beam 26 for removal thereof. In the closed ion source 30 in which the reagent gases enter the source along the ion beam axis 25, the anode 32 is typically an integral part of the ion source 30. The disassembly sequence for replacing the anode 32 requires the removal of a number of component parts including the sealing disk 34, a compression spring (not shown), the heated filament 14, and then the actual anode structure prior to replacement. Replacement of the anode 32 for axial gas entry closed ion sources is therefore a major rework of the ion source assembly. As noted, minimally the anode assembly is replaced but also the filament 14 more than likely also requires replacement. This is especially true if the filament is made from tungsten, due to its brittle nature and the risk of fracture of the filament on assembly. A new (e.g., unheated) tungsten filament is much less brittle than one that has already been heated. Often, a user may opt to replace the complete ion source other than to perform disassembly in the field.